1. Field of the Invention
The invention relates to compositions which are crosslinkable with elimination of alcohols from alkoxysilyl end groups to give elastomers, and which comprise a tin catalyst comprising a tin compound (T) having a coordination number of 5 or 6.
2. Description of the Related Art
Compositions crosslinkable with elimination of alcohols from alkoxysilyl end groups to give elastomers, for example single-component organopolysiloxane compositions crosslinkable with elimination of alcohols to give elastomers (RTV1 alkoxy compositions), need catalysts for the crosslinking process.
The use of dialkyltin(IV) compounds as condensation catalysts in RTV1 and RTV2 silicone rubbers is well known. However, in RTV1 alkoxy compositions these tin compounds have the disadvantage that they likewise catalyze an undesired cleavage of the siloxane chains by traces of alcohol (equilibration). These small amounts of alcohol arise during the compounding process through hydrolysis of the alkoxysilanes used as crosslinking agents, by water entrained by the raw materials. The equilibration process produces monoalkoxy end groups on the polysiloxane chain, and these can no longer be crosslinked. The result is prevention of adequate crosslinking of the composition, with the result that correct use of the material either does not give a vulcanizate or at least gives a vulcanizate with inadequate mechanical stability. The equilibration process drastically reduces storage stability, stated in terms of the period for which the RTV1 alkoxy composition can be stored without discernible loss of its properties.
Similar problems are also found with other systems crosslinkable with elimination of alcohols to give elastomers, for example with polyethers having alkoxysilyl end groups. Here again, the dialkyltin(IV) compounds used as condensation catalysts cause storage stability problems, which are suppressed by a particularly inconvenient preparation process, for example baking the polymer/filler mixtures in vacuo for some hours.
Various methods have been developed to improve storage stability in RTV1 alkoxy compositions. As a first example, EP-A-69 256 attempts to eliminate the traces of alcohol by adding compounds which react irreversibly with the alcohol. These compounds are termed scavengers.
Furthermore, attempts have also been made to prepare scavenger-free mixtures. The titanium catalysts or zirconium catalysts described in U.S. Pat. No. 5,674,936 are particularly suitable for this purpose. However, a decisive disadvantage associated with these catalysts is that they either have a strong tendency toward yellowing or indeed have a yellow intrinsic color. This prevents the preparation of transparent, colorless RTV1 mixtures. However, it is specifically these mixtures which are most in demand, because they have universal application.
The use of compounds having Sn—O—P bonds, obtained from diesters of phosphoric acid, is also known, for example from U.S. Pat. No. 3,525,778. However, the tin catalysts known from the literature give rise to the polymer degradation reactions described above when they are used in polysiloxane compositions. This is seen, for example, in the loss, or retarding, of the crosslinkability of RTV1 products.
A further improvement is provided by the use, described in U.S. Pat. No. 6,162,756, of catalysts having Sn—O—P bonds, obtained by reaction of monoesters of phosphoric acid with organic tin compounds. However, it has been found that the very high acid number of the phosphorus compounds used, resulting from the restriction of the invention to the use of monoesters of phosphoric acid, which have to be used at least in amounts equimolar with the tin compound, or in excess, causes problems with storage stability and adhesion problems, these being particularly evident when the adhesive bonds are aged in water.